Process of producing steel strips suitable for an oxidation-resisting surface coating

ABSTRACT

A process is described for producing steel strips suitable for receiving a surface coating which is resisting to oxidation, in particular tin plating, of medium productivity, that does not require for its execution plants extended of a large areas or involving high economical investments. It comprises, starting from pickled hot strips having a thickness &gt;0.7 mm, preferably obtained from thin-slab plants, a single cold rolling step by passing through not more than three stands of the Sendzimir 6Zhi type for the cold reduction of thickness to less than 0.25 mm and subsequent annealing. It is possible to obtain final thicknesses &lt;0.18 mm both by simple reduction, which only provides subsequent skin-passing and finishing steps, and by double reduction on half-raw strip, which provides subsequent passes for thickness reduction of 30%.

The present invention relates to a process for manufacturing steelstrips adapted to receive a surface coating which is resisting tooxidation by using anti-oxidizing elements such as aluminum, chromium ornickel, copper, etc., and in particular tin for producing tin-plate. Asit is known, for depositing the above-mentioned coating metals use ismade of the electrolytic method or, whenever possible, heat treatmentmethods.

High productivity, large plants of this type are known for whichconsiderably large areas and very burdensome investments are required,while a valid alternative thereto is provided by medium-productivityplants (about 200,000 tons/year), which are suitable for being combinedwith Mini-Mills type plants, and this is the technology which thepresent invention is directed to, rather than to the plants and relevanthigh production processes of known type, with the strips for obtainingthe tin-plate generally produced starting from hot strips having athickness of about 2 mm and subsequently brought to the final thicknessthrough two distinct cold rolling steps, one first of which by means ofbig “High-Four” tandem-type rolling mills with 4-5 stands, eitheraligned with the pickling line, or off-line, as disclosed by patent GB1,027,495, still with the same type and number of stands. The secondcold rolling step occurs after annealing with 2-3 stands of the“High-Four” type to bring the strip to the final thickness of 0.14-0.18mm before tin coating. A plant of this type is schematically shown inFIG. 1 and, as already stated, its main limit is the dimensional one, asit extends over very large areas, thus rendering the investment costsparticularly high.

It is true that could be advantageous to use, as starting material forthe production of tin-plate, hot strips such as those preferablyobtained, even not exclusively, by means of in-line systems of thethin-slab type, such as the one described in the published internationalpatent application No. WO 2004/026497, in the name of the presentapplicant. A diagram of such a system of endless rolling is representedin FIG. 2, where it is possible to recognize, downstream of the castingzone, the continuous casting with liquid core reduction (a), a firstroughing (b) directly at the exit of the continuous casting, aninduction heating furnace (c), a finishing mill (e) and, after a compactrapid cooling (f), a coiling reel of the “carousel” type (g). With aplant of this type in fact it is obtained the production of ultrathinhot strips with thickness of 0.7-1 mm instead of 2 mm, thus renderingsimpler and less burdensome the subsequent processing steps, especiallythe cold rolling required to obtain final thicknesses of 0.14-0.18 mm asrequired by the strips ready for a surface coating, so as to become e.g.tin-plate. However, if cold rolling stands of the “High-Four” type areadopted according to the prior art, the drawbacks relating to the demandof large areas and high investment costs, as above mentioned, would beonly reduced but not overcome.

Object of the present invention is that of providing amedium-productivity process for ultrathin strips adapted to receive anoxidation-resisting surface coating that does not require to be carriedout in plants extended over too large areas, and consequently involvinghigh investments costs.

This object of the present invention is obtained with a productionprocess the steps of which are listed in claim 1.

Further objects, advantageous and features of the process according tothe invention will result more clearly from the following detaileddescription with reference to the annexed drawings in which:

FIG. 1 schematically shows the portions of a high productivity plant andrelevant processing steps for obtaining a steel strip ready to besurface coated, according to the prior art, as above discussed;

FIG. 2 schematically shows a “thin-slab” plant for producing hot stripsas starting material for the subsequent processes, particularlyaccording to WO 2004/026497, as already described above; and

FIG. 3 schematically shows a plant for carrying out the processaccording to the invention, downstream of a thin-slab plant of the typeaccording to FIG. 2.

According to the present invention, as already mentioned above, thestrips used as a substrate for the surface coating, preferably forproducing tin-plate, are ultrathin hot strips of 0.7-1.0 mm thickness,obtained from a “thin-slab” plant, such as of the type schematicallyshown in FIG. 2.

With reference to FIG. 3, the process according to the invention isillustrated in its processing steps downstream of the production of hotrolled strip having the desired thickness as above indicated, beingobtained e.g. according to process and plant of FIG. 2. The strip,having a thickness of 0.7-1.0 mm, after pickling is fed to a coldrolling mill of the Sendzimir 6Zhi type, in particular with three standsas illustrated, which is very compact and consequently needs to occupymore reduced area, thus limiting the investment costs. With said coldrolling step the strip thickness is brought to the desired final valueand thereafter the strip is subjected to either an endless or batchannealing and skin-pass with return to the same cold rolling mill.

It should be noted that, unlike as the High-Four type stands, theSendzimir stands are more compact and allow more sensible reductions, asmuch smaller rolls are used to this effect, whereby the contactcross-section is reduced and the specific pressure, for the same valueof applied force, is higher. On the contrary the prior art had used(while adopting the corresponding processes) plants in which, like inFIG. 1, the hot rolling mills are provided with a plurality of stands ofthe High-Four tandem-type. The reason of which resides in the technicalprejudice suggesting that Sendzimir stands should not be used since, dueto their reduced cross-section of contact, they would have enhanced thegeometrical defects of the strip, instead of reducing the same, such asunder the aspect of parallelism. Therefore it was preferred making useof the much larger cross-section area of contact provided by the workingrolls of big diameter which are typical of the High-Four tandem-typestands which allow to correct the defects of the hot strip. This ofcourse to detriment of the investment burden and of the area requiredfor plant installation.

Such a technical prejudice will be fully removed when considering thatthe starting hot strips which are used according to the presentinvention and manufactured in thin-slab plants according to FIG. 2, arecharacterized by an extremely regular geometric profile with convexitiesor crowns of less than 0.3% and thickness tolerances comparable withthose, of reduced size, of the cold rolled products, as shown in thefollowing table.

Standard tolerance Tolerance of the EN 10031 inventive strip .EN 10051Cold Coils Tolerances for Max. Thickness Hot Coils Normal Reduced hotcoils Crown ≦1.50 +/−0.17 +/−0.11 +/−0.08 +/−0.06 0.03 1.51-2.00 +/−0.17+/−0.13 +/−0.09 +/−0.07 0.04 2.01-2.50 +/−0.18 +/−0.15 +/−0.11 +/−0.100.04 2.51-3.00 +/−0.20 +/−0.17 +/−0.12 +/−0.11 0.05

Furthermore it is noted, still with reference to FIG. 3, that with theprocess according to the invention, the Sendzimir 6Zhi rolling mill withthree stands can be used, starting again from a hot rolled strip havingthickness comprised between 0.7 and 1.0 mm to produce either directlythe tin-plate strips of SR type upon simple reduction, after coldrolling up to 0.14-0.18 mm, annealing and skin-passing, or those of theDR type, half-raw or upon double reduction, preferably but notexclusively cold rolled until a thickness of 0.20-0.25 mm, annealed andthereafter reduced to a thickness 0.18 mm with reductions of about 30%.

1.-4. (canceled)
 5. A process for producing steel strips adapted toreceive an oxidation resisting surface coating, comprising: providing apickled hot strip having a thickness≧0.7 mm; performing a single coldrolling step by passing through not more than three stands of aSendzimir 6Zhi type to obtain a cold reduction of the thickness to lessthan 0.25 mm; and performing a subsequent annealing step.
 6. The processof claim 5, wherein the steel strips are simple reduction (SR) steelstrips, and wherein the single cold rolling step is carried out until athickness≦0.18 mm is reached, and the annealing step is followed only byskin-passing and finishing steps.
 7. The process of claim 5, wherein thesteel strips are half-raw double reduction (DR) steel strips, andwherein the single cold rolling step is carried out until a thicknessbetween 0.20 mm and 0.25 mm is reached, and upon annealing, thethickness is reduced to less than 0.18 mm.
 8. The process of claim 5,wherein the pickled hot strip having a thickness≧0.7 mm is obtainedthrough a thin-slab plant.
 9. The process of claim 5, wherein the steelstrips are half-raw double reduction (DR) steel strips, and wherein thesingle cold rolling step is carried out until a thickness between 0.20mm and 0.25 mm is reached, and upon annealing, the thickness undergoes areduction of about 30%.